1. Field
This invention relates to pivoting means with a self restoring capability particularly suitable for being used in a low cost pivoting print hammer.
2. Description of the Prior Art
A number of apparatuses need low amplitude pivoting means for repetitively pivoting a mechanical load from a given rest position to a given work position and back, with external energy being used to pivot the mechanical load in one direction only, while the pivoting means, provided with self restoring capability, brings the mechanical load back to its rest position upon removal of the external energy. For instance, impact printing is performed by selectively striking a character bearing print element against a print ribbon and a recording medium held by a platen using a pivoting print hammer. The print hammer includes a mechanical load, made of a hammer head attached to a power arm which is pivoted from a rest position to an impact print position for striking a selected character of the print element. A subsequent print operation may then be performed after the hammer head and power arm are returned back to their rest position. The involved pivot for the hammer head and power arm may include a shaft and bearing arrangement which has inherent mechanical clearances and suffers from progressive wear due to the large number of pivotal movements to be achieved during the printer life. The application of lubrication to increase the bearing life is a troublesome, time consuming and costly operation.
Elimination of bearing wear and lubrication requirements has been made possible by resiliently mounting the power arm and hammer head of a printer. U.S. Pat. No. 3,504,623 to K. J. Staller shows such a device wherein a power arm and hammer head arrangement is mounted to a frame by means of two leaf springs. These leaf springs are mounted parallel to each other with one extremity of each of the leaf springs being connected to the frame and the other being connected to the power arm and hammer head arrangement. The power arm and hammer head arrangement is thus made movable from its rest position to an impact print position and back, by transversely flexing the leaf springs and moving their longitudinal axes in a plane parallel to the plane of motion of said power arm and hammer head arrangement. Such a system suffers from three main drawbacks. First, a very efficient additional dampening system must be provided in order to avoid any undesirable oscillations about the rest position of the power arm and hammer head arrangement upon its return from the impact print position. High speed printing performances are then questionable. Second, only limited lateral rigidity can be achieved with this arrangement resulting in undesirable torsional effects occurring within the springs. This leads to low quality printing due to the hammer head missing its target, i.e. the center of gravity of the selected character bearing print element. Third, the large bending displacements of the springs tend to excite the higher modes of vibrations of the springs resulting in premature fatigue failures.